MetService says what appears to be a massive bug swarm is making its way down the North Island.

According to a recent post on its website, the weather agency says people looking at the radar imagery overnight or this morning could be forgiven there was a lot of rain over the northern half of the North Island.

"Except for parts of Gisborne and Hawke's Bay, we know that there was almost no precipitation over the North Island in the period covered by the radar imagery above."

So, what is it then? The MetService says it isn't completely sure, but it's likely to be swarms of insects.

"To show up in radar imagery like this, they must be about as large as, and as numerous as, precipitation particles."

MetService spokesman Dan Corbett says radar imagery is very sensitive and often picks up insect swarms, but this is a particularly good example.

He says MetService can't be 100% sure it is an insect swarm.

"But we've seen these sorts of signatures before and a lot of times that's what it turns out to be."

Mr Corbett isn't sure what kind of insects they are, or whether they are likely to make their way towards land, but he's confident the swarm poses no threat to aircraft.